3.3.
IMO this is in the grey area between feature and bug fix. I think it is possible to argue that it can be treated as a bug fix, but I think we need opinions from other developers if we want to try to go that route.
The reason I think it can be argued that it can be treated as a bug fix is that (if I understand correctly) there is no difference in the application code. The only difference is whether or not one can successfully communicate with IIS 6. That may not be a sufficient argument, but it is an argument :)